1. Field of the Invention
The invention relates to a transmitter with a power amplifier that amplifies a radio frequency transmit signal that is a non-constant envelope modulated signal.
The invention further relates to a portable apparatus including such a transmitter, to a module including such a transmitter, and to a method for sliding a compression point in such a transmitter. Such transmitters may be used in CDMA systems referred to as linear modulation systems, or any other suitable device or system with non-constant envelope modulated signals requiring linear modulation, in which information is carried as phase and amplitude modulated information.
2. Description of the Related Art
In U.S. Pat. No. 5,936,464 a method for reducing distortion in a high efficiency power amplifier is disclosed. The power amplifier is included in a communication device that transmits signals having information in both amplitude and phase. Such transmission requires low distortion to be achieved by so-called linear modulation techniques. Non-linear operation, in addition to distortion and loss of information, causes an increase in intermodulation products that cause undesirable amounts of energy to be present in frequency bands other than the one intended for use. This undesirable energy is commonly quantified and termed Adjacent Channel Power (ACP), or referred to as Adjacent Channel Power Ratio (ACPR). Driving the power amplifier into saturation increases efficiency of the power amplifier but also causes intermodulation products to rise, so a tradeoff is needed between high efficiency and low intermodulation products. High efficiency is of particular importance when the transmitter is used in a battery operated portable apparatus, for saving battery power. In U.S. Pat. No. 5,936,464, in a so-called Envelope Elimination and Restoration (EER) type amplifier, efficiency is gained by operating the final power amplifier at or near saturation. When operating at or near saturation, the amplifier is operating at a region of compression, where as drive levels change, the phase of the output also changes. Even though the amplifier is constantly at or near saturation, the compression point is changing as the drain bias of the final power amplifier changes. U.S. Pat. No. 5,936,464 provides a strategy for minimizing phase errors in a power amplifier which are a function of changes in the compression point. In U.S. Pat. No. 5,936,464 though, the power amplifier is operated at a substantially constant compression point so as to minimize phase error contributions from the final power amplifier.
It is an object of the invention to provide a transmitter that operates at a backoff from a power amplifier compression point for an average input signal level of a non-constant envelope modulated signal input to a power amplifier comprised in the transmitter, while still meeting a given adjacent power ratio requirement for a peak-to-average ratio of a radio frequency transmit signal, for different information content of the non-constant envelope modulated signal.
It is another object of the invention to provide such a transmitter that is configurable to transmit information content in accordance with different radio configurations.
To this end, a transmitter is provided comprising:
power amplifier means for amplifying a radio frequency transmit signal, said power amplifier means having a first compression point, said radio frequency transmit signal being a non-constant envelope modulated signal; operating means for operating said power amplifier means at a first backoff from said first compression point such that a given adjacent channel power ratio requirement is met for a first peak-to-average ratio of said radio frequency transmit signal; and sliding means for sliding said first compression point to a second compression point for a second peak-to-average ratio of said radio frequency transmit signal, said second compression point being lower than said first compression point, said first and second peak-to-average ratios being dependent on information content of said non-constant envelope modulated signal.
In a preferred embodiment, the compression point is slide so as to just meet the ACPR requirement, for the new compression point, thereby achieving maximal power savings, the same ACPR is obtained for a lower peak-to-average ratio of the modulated signal at the new compression point than the ACPR for the higher peak-to-average ratio of the modulated signal at the old compression point, the peak-to-average ratios being dependent on the information content of the non-constant envelope modulated signal. In other embodiments, the compression point may be slide to ACPRs that are higher at the new compression point that at the old compression point, thereby still achieving power savings.
In an embodiment, the information content is determined by a current radio configuration according to which the transmitter transmits the radio frequency signal. In CDMA systems, such radio configurations include the number of channels, the data rate per channel, the orthogonal code per channel, the type of encoder to be used, and other parameters that are related to the information content of the signal to be transmitted. The transmitter may be configured to transmit signals that have information content in accordance to different radio configurations, and therefore have different peak-to-average ratios. The compression point is set in accordance with these different radio configurations.
In an embodiment, the different radio configurations and related peak-to-average ratios are included in a look-up table, and the compression point is set in accordance with a current radio configuration, thereby using the look-up table. For multi-standard operation of the transmitter, the look-up table may be multi-dimensional, with separate entries for different standards. In that embodiment, the transmitter may have different power amplifiers for different standards that are respectively powered-up for the transmitter operating in accordance with a current radio standard, the other power amplifiers being powered down.
In an embodiment, the compression point may be slide by adjusting a biasing current in the power amplifier.